This invention pertains generally to the field of plasma processing of materials and particularly to plasma treatment of seeds.
Seeds produced by commercial seed companies are commonly treated with insecticides and fungicides to enhance the survivability and germination rate of the planted seed. The fungicides and pesticides may be applied to the seed in a dry or wet form. A dry treatment involves application of the active ingredient in an inert dust which may contain additives to prevent agglomeration of the particles or xe2x80x9cstickersxe2x80x9d to enhance adhesion to the seed surface. Adhesion of dry particles to the seed surface is a complex process which involves molecular forces and physical trapping of small particles. Both molecular forces and physical trapping of particles are strongly dependent upon the particle size. Molecular forces of adhesion are very high per unit area and essentially depend on the surfaces that are in actual contact. Rough surfaces have low contact areas and, as a consequence, molecular forces generally play a less important role in adhesion between such surfaces. Physical entrapping of active particles is also related to the particle size. The porosity of the seed surfaces should be comparable with the average particle size to obtain efficient trapping of particles to the seed.
Wetting agents can also be used to allow powdery active materials to be applied to seeds using a slurry treatment. Such treatments usually are performed under low liquid volume conditions, but still have the disadvantage that the seeds generally must be dried afterwards, which increases the expense of the treatment process.
The result of such treatments, in whatever manner performed, is a fairly high concentration of active ingredients on the seed surfaces which, of course, enhances the utility of these seeds when planted in the normal course. However, for a variety of reasons, large quantities of treated seed either are not or cannot be used for planting within an appropriate time after the seeds have been treated. Often, seed companies treat more seeds than are expected to be used during a planting season to-ensure the availability of seeds in a subsequent season if there is an intervening crop failure. In most years, the additional seed is not planted. Over long storage times, the active ingredients in the surface treatments may degrade, leading to the formation of secondary compounds which are not active for the intended purpose of the surface treatment. However, such contaminants may present fairly high toxicity levels. Thus, such overaged treated seed is not acceptable for use as seeds for planting or for human or animal feed. Wet-chemical removal of fungicide, pesticide or insecticide contaminants from seeds would require large quantities of liquids (water, organic solvents) and expensive drying technologies. In addition, the combination of long storage times and liquid treatments may enhance the penetration of the surface borne chemicals into the seeds, potentially damaging or killing the seeds. Seeds with cracks or exposed embryos, such as from thresher damage or ventilation, may absorb even higher quantities of the surface borne toxins.
In accordance with the present invention, seeds are treated by exposing the seeds to a cold plasma to etch the surface of the seeds to remove surface materials, such as fungicide and insecticide chemicals, and/or to disinfect the surface. The cold plasma etching process may be carried out employing etch gases which are not themselves harmful to the seeds, and for selected periods of time sufficient to remove surface materials such as chemicals or other contaminants without significantly affecting the viability of the live seeds.after treatment. The plasma treatment process can be carried out .under conditions allowing removal of a selected thickness of surface material from the seeds with precision. In addition, because the plasma treatment process is carried out under dry conditions, no additional moisture need be added to the seeds during the treatment process, and moisture may even be removed from the seeds during the plasma treatment.
A cold plasma process in accordance with the invention has several advantages over liquid-based treatment processes for detoxification of seeds or removal of surface materials from seeds. Because large quantities of liquids, such as solvents, are not needed, and only small amounts of gas-phase materials are required, the process has much less environmental impact than liquid-based processes. Because of the low pressures under which the plasma reactions develop, minimal quantities of the plasma gases are required to sustain the plasma discharge. The plasma processing conditions can be selected so that the plasma species penetrate and interact only with the very top layers of a seed, leaving the bulk of the seed unaffected. The plasma species can interact efficiently with the surface layer molecules, and regardless of the nature of such molecules, molecular fragmentation (etching) of the surfaces can take place. Various plasma process parameters, such as power coupled to the plasma, gas pressure, and treatment time, can be selected to tailor the etch rate and the nature of the gas-phase components that result after the treatment. The molecules or molecular fragments resulting from the etching process usually are gas-phase components which can easily be removed from the system. Depending on the nature of the plasma gases employed, which may be inert or reactive gases (e.g., argon, CF4, air, oxygen, water vapor, etc.), the etch rates and the chemical nature of the resulting volatile components (toxic or non-toxic derivatives) can also be controlled and tailored to specific process requirements. The plasma generated gas-phase components that result from the process can be easily trapped and disposed of if they constitute hazardous waste or, if non-hazardous, may be released to the environment.
In a preferred method of treating seeds in accordance with the invention, the seeds to be treated are enclosed in a reaction chamber, the reaction chamber is evacuated to a base level, and a selected source gas is supplied to and a selected pressure established in the reaction chamber. The gas may be provided from an external gas source and is selected to yield a desired etch characteristic and not a deposit during the processing. The gas may constitute water vapor emitted from the seeds themselves as the pressure within the reaction chamber is reduced below atmospheric. Further, multi-step processes may be carried out. For example, an initial cold plasma may be ignited in the water vapor evolved from the seeds, and the seeds may be exposed to this plasma for a selected period of time. An external gas may then be introduced into the reaction chamber and the cold plasma ignited in the external source gas. The gas in the chamber may be ignited by coupling RF power to the gas in the chamber in various ways, including capacitive coupling and inductive coupling. In addition, the RF power may be coupled in pulses to the plasma in the reaction chamber.
Virtually any type of seed can be treated in accordance with the present invention. The invention has particular application to seed corn which is conventionally treated with insecticides and fungicides. After treatment in accordance with the present invention to plasma etch the surface borne chemicals therefrom, the corn or other seeds may be used for animal feed, or may be retreated with insecticides, etc. at a later time so that the seeds will be properly treated for use in a later growing season.
Gases that may be employed in accordance with the present invention may be any of the various reactive gases which will provide plasma etching in a cold plasma process. For example, gases including, but not limited to, argon, CF4, air, oxygen, water vapor, and mixtures thereof may be used in the process.
Cold plasma treatment in accordance with the present invention may also be employed to reduce the amount of extraneous flakes and dust intermixed with the seed by physical ablation of such materials or by oxidation, thus providing a cleaner bulk seed product after treatment with less dust (possibly carrying toxic particles) than is typically the case with normal bulk seeds.
A cold plasma etching process in accordance with the invention may also be carried out on either treated or untreated seed, to remove all or part of the surface layers of the seed for various purposes, including affecting the germination rates of the seed by, e.g., changing the water absorption characteristics of the seed surfaces. The plasma etching process may be carried out to remove selected depths of the surface layers of the seed, including, if desired, entire removal of the pericarp.
Further objects, features, and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.